During the past several years, substantial improvements have been made in three dimensional (3D) seismic surveys to better locate and define the boundaries of underground hydrocarbon producing formations. During this same time period, substantial improvements have also been made in directional drilling and horizontal well completion techniques. As a result, many current well completions often include more than one wellbore or borehole. For example, a first, generally vertical wellbore may be initially drilled within or adjacent to one or more hydrocarbon producing formations. Multiple wellbores may then be drilled extending from the vertical wellbore to selected locations designed to optimize production from the hydrocarbon producing formation or formations. Such well completions are often referred to as multilateral wells.
A typical multilateral well completion will include a generally vertical or primary wellbore defined in part by a casing string and a layer of cement disposed between the exterior of the casing string and the inside diameter of the primary wellbore. Directional drilling equipment and techniques may be used to form an exit or window in the casing string and layer of cement at a downhole location selected for drilling a lateral or secondary wellbore from the primary wellbore. The location of the window from the primary wellbore, the orientation of the window, the length and diameter of the secondary wellbore and the orientation of the secondary wellbore relative to the primary wellbore and the hydrocarbon producing formation are selected based on characteristics of the associated hydrocarbon producing formation. For many locations such as deep offshore wells, multiple secondary or lateral wellbores will be drilled from each vertical wellbore in an effort to optimize hydrocarbon production while minimizing drilling and completion costs. Selective isolation and/or reentry into each of the secondary or lateral wellbores is often necessary to further optimize production from the associated hydrocarbon producing formations or formation.
A typical multilateral well completion will have one or more production tubing strings disposed within the casing string of the primary wellbore. The production tubing string or strings will have a generally uniform inside diameter extending from the well surface to a selected downhole location. A reentry window assembly, sometimes referred to as a lateral reentry window, will be installed within each production tubing string at a downhole location corresponding with the location at which a secondary or lateral wellbore intersects the primary wellbore. For example, a multilateral well completion may have a first wellbore or primary wellbore with three secondary or lateral wellbores intersecting the primary wellbore at respective first, second and third downhole locations. A production tubing string with three window assemblies may be installed within the casing string of the primary wellbore using conventional well completion techniques such that each window assembly is disposed adjacent to a respective lateral or secondary wellbore.
In order to deflect well tools from the primary wellbore into a preselected lateral wellbore, a tubing exit whipstock is installed within the primary wellbore at a location adjacent to the preselected lateral wellbore. The surface of the tubing exit whipstock is tapered toward the lateral wellbore to provide a smooth transition. Existing tubing exit whipstocks include a semitubular neck attached to the whipstock at one end and provide a collar with a grooved profile for connection to an installation for retrieving a tool at another end. The diameter of the collar containing the grooved profile limits the diameter of any downhole tool which must pass through the collar to enter the lateral wellbore.